The uninterruptible power supply apparatus comprises an AC switch for turning on/off the AC power that is input from a commercial AC power source; a storage battery; a power conversion device; a load device connected to the output end of the AC switch; an output AC voltage detection unit for detecting an output AC voltage that is output from the AC switch; and an abnormality detection circuit for detecting that the detected output AC voltage is outside predefined upper and lower limits; wherein said AC switch turns off in accordance with a signal detected by said abnormality detection circuit. Further comprises a restoration circuit for generating a restoration signal when the AC voltage detected by an input AC voltage detection unit exceeds at least a lower-limit AC voltage setting that is increased gradually or continuously in accordance with a load factor computed by a load factor computation circuit.